Plasma doping method

ABSTRACT

A plasma doping method that can control a dose precisely is realized. In-plane uniformity of the dose is improved. 
     It has been found that, if a bias is applied by irradiating B 2 H 6 /He plasma onto a silicon substrate, there is a time at which a dose of boron is made substantially uniform, and the saturation time is comparatively long and ease to stably use, compared with a time at which repeatability of an apparatus control can be secured. The invention has been finalized focusing on the result. That is, if plasma irradiation starts, a dose is initially increased, but a time at which the dose is made substantially uniform without depending on a time variation is continued. In addition, if the time is further increased, the dose is decreased. The dose can be accurately controlled through a process window of the time at which the dose is made substantially uniform without depending on the time variation.

BACKGROUND OF THE INVENTION

1. Background of the Invention

The present invention relates to a plasma doping method, and inparticular, to a plasma doping method for doping an impurity into asurface of a solid sample such as a semiconductor substrate.

2. Description of the Related Art

As a technology for doping an impurity into a surface of a solid sample,a plasma doping (PD) method for ionizing the impurity and doping theionized impurity into a solid at low energy is well known (for example,see U.S. Pat. No. 4,912,065).

On the other hands, among the methods for doping an impurity, an ionimplantation method is most widely used at present. The plasma dopingmethod is described in “Column of Shallow Junction Ion Doping of FIG. 30of Front End Process in International Technology Roadmap forSemiconductors 2001 Edition (ITRS2001)” and “International TechnologyRoadmap for Semiconductors 2003 Edition (ITRS2003)” as a next-generationtechnology for implanting ion. The plasma doping method is differentfrom the ion implantation method. Moreover, ITRS is a document that iswidely referred to by engineers in semiconductor industries. A technicaldifference between ion implantation and plasma doping will now bedescribed in more detail.

In the ion implantation method, an apparatus comprising an ion sourcefor generating plasma from gas, an analysis magnet for performing massseparation in order to select desired ions among ions extracted from theion source, an electrode for accelerating the desired ions, and aprocess chamber for implanting the accelerated desired ions into asilicon substrate, is used. In the ion implantation, in order to implantthe impurity shallow, it is preferable to set extraction energy forextracting ions from the ion source and acceleration energy foraccelerating small. However, when the extraction energy is set small,the number of ions to be extracted is decreased. In addition, when theacceleration energy is set small, while an ion beam is transported fromthe ion source to a wafer, a beam diameter is widened due to a repulsiveforce generated by charges between the ions. As a result, the ion beammay collide against the inner wall of a beam line, and a large number ofions may be lost. For this reason, throughput of an implantationprocessing will be lowered. For example, when B+ ions are implanted, ifthe acceleration energy becomes 2 keV or less, the throughput starts tobe lowered, if the acceleration energy becomes 0.5 keV or less, the beamtransportation itself become difficult. Further, even though theacceleration energy is lowered to 0.5 keV, the B ions may be implantedat a depth of approximately 20 nm. That is, in case of forming anextension electrode having a thinner thickness than the depth,productivity may be lowered drastically.

In contrast, in the plasma doping method, an apparatus comprising aplasma generation source for inducing plasma into a cylindrical vacuumchamber in which a silicon substrate can be disposed, a bias electrodeon which the silicon substrate is disposed, and a bias power supply foradjusting a potential of the bias electrode, is used. The apparatus hasthe different configuration from the apparatus used in the ionimplantation in point that the analysis magnet and the accelerationelectrode are not provided. The bias electrode serving as a plasmasource and a wafer holder is provided in the vacuum chamber. Then, theions are accelerated and introduced by a potential to be generatedbetween the plasma and the wafer. With this configuration, sincelow-energy plasma can be directly used, a large amount of low-energyions can be irradiated onto the wafer, compared with the ionimplantation. That is, a dose rate is considerably high. For thisreason, in the low-energy B-ion implantation, high throughput can bekept.

By applying the plasma doping method, the inventors have developed aprocess technology for forming a source-to-drain extension electrodehaving a very shallow thickness and low resistance. The paper on thisnew process technology is adopted in VLSI Symposium that is the highestauthority as one of International Conferences, in June 2004. This newprocess technology is known as a process technology that has particulareffects (“Y. Sasaki, et al., Symp. on VLSI Tech. p 180 (2004)”).

In this method, doping material gas which is introduced from a gasintroduction port, such as B₂H₆, is plasmized by a plasma generationunit having a microwave waveguide and an electric magnet. Then, boronions in plasma are supplied to a surface of a sample by a high-frequencypower supply.

With the reduction in size and high integration of a semiconductordevice, characteristics in an impurity doped region are very important.Among these characteristics, a dose (impurity doping amount) determineslow resistance that is one of important elements in determining elementcharacteristics. Accordingly, the control of the dose is very important.

If the plasma doping method is used, it can be seen that thesource-to-drain extension electrode having a very shallow thickness andlow resistance can be formed. However, a dose control method forcontrolling the element characteristics has not been developed yet. Upto now, a method for changing the dose by way of changing a plasmadoping time has been tested, but this method does not obtain sufficientcontrol precision, and as a result, it is unpractical.

In this situation, as a method which is capable of improving safety bydiluting toxic B₂H₆ having a serious risk to the human body as large aspossible, stably generating and keeping plasma without degrading dopingefficiency, and easily performing the control of the dopant dose, theinventors has suggested the following method. In the method, B₂H₆ gas asa material containing an impurity to be doped is diluted with He gashaving small ionization energy, then He plasma is generated earlier, andsubsequently B₂H₆ is discharged (JP2004-179592) suggested that theconcentration of B₂H₆ gas is preferably less than 0.05%.

When the concentration is low, for example, approximately 0.05%,although it is reported that the dose is easily controlled, changing thedose by varying the time while the gas concentration is kept constant isdescribed. That is, when the B₂H₆ gas concentration is low, the changein the dose is small with respect to the variation in time, and thus thedose is easily controlled. Here, there is a progress in that the controlprecision of the dose is increased. However, this just improves theknown method for changing the dose by changing a plasma doping time.Herein, there is no study for the relationship between the change in thedose and the gas concentration.

As described above, it is important to form the impurity doped region orcontrol the dose. Of course, in-plane uniformity is also very importantto form an element. In particular, while there is a recent progress in alarge diameter of a wafer, it is very difficult to obtain a uniform dosein the surface.

SUMMARY OF THE INVENTION

The invention has been finalized in consideration of the above problems,and it is an object of the invention to provide a plasma doping methodthat can control a dose with high accuracy and can form a shallowimpurity doped region.

It is another object of the invention to provide a plasma doping methodthat can realize high in-plane uniformity of a dose and can control thedose with high accuracy.

According to an aspect of the invention, there is provided a plasmadoping method that supplies gas plasma containing impurity ions to asample for a predetermined time with a predetermined concentration so asto form an impurity doped region in a surface of the sample. The plasmadoping method includes a step of setting a doping time and aconcentration of the gas plasma containing the impurity such that a doseis made uniform with no time dependency.

The inventors have repeated various experiments and have found that, ifa bias is applied by irradiating B₂H₆/He plasma onto a siliconsubstrate, there is a time at which a dose of boron is madesubstantially uniform. Further, it can be seen that the saturation timeis comparatively long and ease to stably use, compared with a time atwhich repeatability of an apparatus control can be secured. That is, ifplasma irradiation starts, a dose is initially increased, but a time atwhich the dose is made substantially uniform without depending on a timevariation is continued. In addition, if the time is further increased,the dose is decreased. The inventors have also found that the dose canbe accurately controlled through a process window of the time at whichthe dose is made substantially uniform without depending on the timevariation. The invention has been finalized focusing on the results.

According to another aspect of the invention, there is provided a plasmadoping method that supplies gas plasma containing impurity ions onto asample for a predetermined time with a predetermined concentration so asto form an impurity doped region in a surface of the sample. The plasmadoping method includes a step of performing plasma doping within a timerange in which a dose is made uniform with no time dependency in a statewhere a doping time and a concentration of the gas plasma containing theimpurity ions are set such that a dose is made uniform with no timedependency.

According to this configuration, the concentration of the gas plasmacontaining the impurity ions is set in the time range in which the doseis made uniform with no time dependency. Accordingly, the dose can beaccurately controlled. That is, even though the time is slightlychanged, since the dose is not almost changed, the dose can be stablycontrolled. In contrast, in the known method that controls the doseaccording to the time, even though the time is slightly changed, thedose is drastically changed.

As a result of the repetitive experiment, when the B₂H₆/He concentrationis changed with respect to a predetermined bias, it is possible toobtain a time range where the dose is not almost changed and issaturated. In addition, the inventors have found that, in the above timerange, there is a time range where in-plane uniformity of sheetresistance (Rs) after annealing, that is, in-plane uniformity of thedose is very satisfactory. The invention has been finalized on the basisof the result. Accordingly, while plasma doping can be put to practicaluse, the problems relative to the control of the dose and in-planeuniformity can be solved by one effort.

In the plasma doping method according to another aspect of theinvention, the doping time and the concentration of the gas plasmacontaining the impurity may be set such that doping of the impurity intothe surface of the substrate and sputtering of the impurity from thesurface of the substrate are saturated.

The inventors have repeated various experiments and have found that, ifa bias is applied by irradiating the B₂H₆/He plasma onto the siliconsubstrate, there is a time at which doping of boron and sputtering ofboron from the surface of the substrate by irradiation ions, radicals,gas in the plasma are saturated (balanced). Further, it can be seen thatthe saturation time is comparatively long and ease to stably use,compared with a time at which repeatability of an apparatus control canbe secured.

In the plasma doping method according to another aspect of theinvention, the doping time may be set to an extent that a dose in aportion where the dose in the surface of the substrate is small followsa dose of a portion where the dose is saturated.

According to this configuration, the concentration of the plasmacontaining the impurity ions is set in the time range in which the doseis made uniform with no time dependency. Further, the doping time is setto an extent that the dose in the portion where the dose in the surfaceof the substrate is small follows the dose of the portion where the doseis saturated. Accordingly, in order to increase in-plane uniformity, thedose can be accurately controlled. Actually, the saturation time rangeis measured at individual points in the surface, and a doping end pointis set to the latest time among start points of the measured time range.Accordingly, the dose in the portion where the dose in the surface ofthe substrate is small follows the dose in the portion where the dose issaturated. Therefore, satisfactory in-plane uniformity can be obtained.

In the plasma doping method according to another aspect of theinvention, the level of the uniform dose with no time dependency may bechanged as the concentration of the gas plasma containing the impurityions is changed.

With this configuration, the concentration of the gas plasma is set suchthat doping of the impurity ions into the surface of the substrate andsputtering from the surface of the substrate are saturated. Accordingly,the level of the uniform dose can be changed. Therefore, it is possibleto form an impurity region in which an impurity concentration is stablycontrolled with high accuracy.

In the plasma doping method according to another aspect of theinvention, the concentration of the gas plasma containing the impurityions as a concentration and a pressure of gas containing impurity atomsand source power are changed.

With this configuration, the concentration of the gas plasma can be setin a desired range.

In particular, when the concentration of the gas containing the impurityatoms is changed, the concentration of the gas plasma containing theimpurity ions can be simply and accurately changed. As a specific unit,the gas including the impurity atoms and a dilution gas may be mixed tobe then used, and flow rates of the gases may be changed by mass flowcontrollers, such that mixture ratios of the gases are changed.Therefore, the concentration of the gas plasma containing the impurityions can be simply and accurately changed.

In the plasma doping method according to another aspect of theinvention, the concentration of the gas plasma, and the concentrationsof ions, radicals, and gas of the gas plasma may be set such that dopingof the impurity ions into the surface of the substrate and sputteringfrom the surface of the substrate are saturated as time increases.

With this configuration, the concentration of the gas plasma is set suchthat doping of the impurity ions into the surface of the substrate andsputtering from the surface of the substrate are saturated. Accordingly,the dose can be accurately controlled without depending on the timevariation. Therefore, it is possible to form an impurity region in whichthe impurity concentration is stably controlled with high accuracy.

In the plasma doping method according to another aspect of theinvention, the gas plasma containing the impurity ions may be mixturegas plasma of molecules (B_(n)H_(m)) having boron atoms and hydrogenatoms.

The inventors have repeated various experiments and have found that,when a concentration of B_(n)H_(m) gas is being small, there is a regionwhere the dose is made substantially uniform without depending on thetime variation. Then, the concentration of the B_(n)H_(m) gas is setsuch that doping of boron into the surface of the substrate andsputtering from the surface of the substrate are saturated. Accordingly,the dose can be accurately controlled without depending on the timevariation. Therefore, it is possible to form the impurity region inwhich the dose of the impurity is stably controlled with high accuracy.

In the plasma doping method according to another aspect of theinvention, the gas plasma containing the impurity ions may be mixturegas plasma of B₂H₆ and He.

The inventors have made various experiments and have found that, in acase where the mixture gas plasma of B₂H₆ and He is used, when theconcentration of the B₂H₆ gas is being small, there is a region wherethe dose is made substantially uniform without depending on the timevariation. The invention has been finalized focusing on the fact. Then,the concentration of the B₂H₆ gas is set such that doping of boron intothe surface of the substrate and sputtering from the surface of thesubstrate are saturated. Accordingly, the dose can be accuratelycontrolled without depending on the time variation. Therefore, it ispossible to form the impurity region in which the impurity concentrationis stably controlled with high accuracy.

In the plasma doping method according to another aspect of theinvention, a concentration of B₂H₆ gas in the mixture gas plasma of B₂H₆and He may be in a range of 0.01% to 1%. In case of less than 0.01%,when the concentration of B₂H₆ is changed, the change in the dose ofboron to be saturated according to the time variation is excessivelysmall. Accordingly, it is difficult to control the dose of boron to besaturated according to the time variation by the change in theconcentration of B₂H₆. Further, in case of more than 1.0%, when theconcentration of B₂H₆ is changed, the change in the dose of boron to besaturated according to the time variation is excessively large.Accordingly, in this case, controllability is degraded. For the samereason, the concentration of B₂H₆ is more preferably in a range of0.025% to 0.6%.

From the experiment results, it has been found that, when theconcentration of B₂H₆ of the mixture gas plasma of B₂H₆ and He isapproximately 0.1%, there is a region where the dose is madesubstantially uniform without depending on the time variation.

In the plasma doping method according to another aspect of theinvention, a bias voltage V_(DC) may be 60 V or less.

From the experiment results, it has been found that, when the biasvoltage V_(DC) is 60 V or less, there is a region where the dose is madesubstantially uniform without depending on the time variation.

In the plasma doping method according to another aspect of theinvention, source power may be approximately 1500 W.

From the experiment results, it has been found that, when source poweris approximately 1500 W, there is a region where the dose is madesubstantially uniform without depending on the time variation.

In the plasma doping method according to another aspect of theinvention, the gas plasma containing the impurity ions may be mixturegas plasma of BF₃ and He.

From the experiment results, like a case where the mixture gas plasma ofB₂H₆ and He is used, in a case where the mixture gas plasma of BF₃ andHe is used, it has also been found that, when the concentration of BF₃gas is being small, there is a region where the dose is madesubstantially uniform without depending on the time variation.Therefore, it is possible to form the impurity region in which the doseof the impurity is stably controlled with high accuracy.

In the plasma doping method according to another aspect of theinvention, the sample may be a silicon substrate.

From various experiment results, in a case where the mixture gas plasmaof B₂H₆ and He is used upon doping into the silicon substrate, it hasbeen found that, when the concentration of B₂H₆ gas is being small,there is a region where the dose is made substantially uniform withoutdepending on the time variation. When the mixture gas plasma ofB_(n)H_(m) and He is used, the same effects can be obtained.

As such, in the invention, the doping time and the concentration of thegas plasma containing boron is set such that doping of boron into thesurface of the substrate and sputtering from the surface of thesubstrate.

In the plasma doping method according to another aspect of theinvention, plasma doping may be performed in the time region where thedose is made uniform with no time dependency. Accordingly, it ispossible to form the impurity region in which the dose of the impurityis stably controlled with high accuracy.

With this configuration, the dose can be accurately controlled through aprocess window of the time at which the dose is made uniform withoutdepending on the time variance.

Preferably, the activation step may include a step of irradiating laserlight.

Since laser light has a high energy density, high-efficient activationcan be performed.

The activation step may include a step of irradiating radiation light ofa flash lamp.

The flash lamp is cheap, and thus low cost can be realized.

The activation step may include a step of irradiating radiation light ofa tungsten halogen lamp.

All the heat treatments using the tungsten halogen lamp have been put topractical use, and thus the activation can be performed with goodreliability.

In the plasma doping method according to another aspect of theinvention, the plasma doping may be performed in a state where atemperature of an inner wall of a reaction chamber coming into contactwith the plasma is made substantially uniform.

In the plasma doping method according to another aspect of theinvention, plasma doping may be performed in a state where the innerwall of the reaction chamber coming into contact with the plasma isheated.

In the plasma doping method according to another aspect of theinvention, plasma doping may be performed in a state where the innerwall of the reaction chamber coming into contact with the plasma iscooled.

In the plasma doping method according to another aspect of theinvention, the concentration of the gas containing the impurity atomsmay be lowered during a treatment.

As described above, according to the plasma doping method of theinvention, the impurity doping amount can be accurately controlled.Further, it is possible to form the impurity region in which the dose isstably controlled with high accuracy.

Further, it is possible to form the impurity region having excellentin-plane uniformity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an impurity doping apparatus that isused in a first embodiment of the invention;

FIG. 2 is a cross-sectional view of an impurity doping apparatus that isused in a second embodiment of the invention;

FIG. 3 is a diagram showing the relationship between a dose and a timein a method according to Example 1 of the invention;

FIG. 4 is a diagram showing the relationship between a dose and a gasconcentration in the method according to Example 1 of the invention;

FIG. 5 is a diagram showing an Rs distribution of a sample obtained inthe method according to Example 1 of the invention;

FIG. 6 is a diagram showing an Rs distribution of a sample obtained inthe method according to Example 1 of the invention;

FIG. 7 is a diagram showing the relationship between a dose and a timein a method according to Example 2 of the invention;

FIG. 8 is a diagram showing in-plane uniformity of a dose in the methodaccording to Example 2 of the invention;

FIG. 9 is a diagram showing a sheet resistance distribution on a Y axisin the method according to Example 2 of the invention; and

FIG. 10 is a diagram showing a distribution of a sheet resistancestandard value of a Y axis in the method according to Example 2 of theinvention;

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the invention will be described withreference to the drawings.

First Embodiment

Hereinafter, a first embodiment of the invention will be described indetail with reference to the drawings.

An apparatus shown in FIG. 1 is referred to as a plasma doping apparatusA (PD apparatus A).

In this embodiment, upon doping of an impurity, a concentration of gasplasma containing impurity ions to be doped is set such that doping ofthe impurity ions and sputtering of a surface of a silicon substrate(silicon wafer) are saturated. According to this, a dose can beaccurately controlled without depending on a time variation. Therefore,it is possible to form an impurity region in which an impurityconcentration is stably controlled with high accuracy. Further, it ispossible to form an impurity region having excellent in-planeuniformity.

Here, the concentration of the gas plasma is set such that doping of theimpurity ions and sputtering of the surface of the substrate aresaturated. According to this, the dose can be accurately controlledwithout depending on the time variation. Therefore, it is possible toform an impurity region in which a dose of the impurity is stablycontrolled with high accuracy.

This may be expressed as follows. Plasma doping is performed in a timerange where the dose is made uniform, irrespective of the timevariation, when doping of the impurity ions and sputtering of thesurface of the substrate are saturated.

In order to obtain in-plane uniformity, a time range to be saturated ismeasured at individual points in the surface, and a doping end point isset to the latest time among start points of the measured time range.Accordingly, a dose in a portion where the dose in the surface of thesubstrate is small follows a dose in a portion where the dose issaturated, thereby obtaining in-plane uniformity.

FIG. 1 is a cross-sectional view of an impurity doping apparatus used ina first embodiment of the invention.

The impurity doping apparatus, that is, a process chamber includes areaction chamber 15, a turbo molecular pump 6 serving as an exhaustdevice for exhausting the reaction chamber 15, a pressure regulatingvalve 7 serving as a pressure control device that controls a pressure inthe reaction chamber 15, a coil and antenna 3 serving as a plasma sourceprovided in the vicinity of a dielectric window facing a lower electrode14, a high-frequency power supply 12 for supplying high-frequency powerof 13.56 MHz to the coil or antenna 3, and a high-frequency power supply1 serving as a voltage source supplied a voltage to the lower electrode14. A substrate to be processed (substrate) 13 is placed on the lowerelectrode 14 serving as a sample table, and plasma irradiation isperformed onto the substrate 13.

Here, a high frequency is supplied from the coil and antenna 3 throughthe high-frequency power supply 1 for generating plasma and the matchingbox 2 for adjusting the discharge. Required gas is supplied through themass flow controllers (MFC) 4 and 5. A degree of vacuum in the reactionchamber 15 is controlled by the mass flow controllers 4 and 5, the turbomolecular pump 6, the pressure regulating valve 7, and the dry pump 8.Power is supplied to the reaction chamber 15 from the high-frequencypower supply 12 through the matching box 11. The substrate 13 to beprocessed that is provided in the reaction chamber 15 is placed on thesample table 14, and then the power is supplied.

Next, a plasma doping process will be described.

A predetermined gas is introduced from the gas supply device into thereaction chamber 15 of the process chamber through the mass flowcontrollers 4 and 5, while gas exhaust is performed by the turbomolecular pump 6 as an exhaust device. Further, the reaction chamber 15is kept at a predetermined pressure by the pressure regulating valve 7as a pressure control device. Then, high-frequency power of 13.56 MHz issupplied from the high-frequency power supply 1 to the coil 3 as aplasma source, such that inductively coupled plasma is generated in thereaction chamber 15. In this state, high-frequency power is supplied bythe high-frequency power supply 12, and then the potential of the lowerelectrode 14 can be controlled such that the silicon substrate (thesubstrate to be processed) 13 as a sample has a negative potential withrespect to plasma.

Although an inductively coupled plasma is generated using the coil inthis embodiment, an antenna may be used instead of the coil.Alternatively, helicon plasma, surface wave plasma, electronic cyclotronresonance plasma, or the like may be generated.

After the silicon substrate 13 is placed on the sample table 14 as thelower electrode, while the reaction chamber 15 is exhausted, helium gasis supplied into the reaction chamber 15 by the mass flow controller 4and diborane (B₂H₆) gas as doping material gas is supplied into thereaction chamber 15 by the mass flow controller 5. At this time, thepressure regulating valve 7 is controlled such that the pressure of thereaction chamber 15 is kept at 0.9 Pa. Next, high-frequency power 1500 Wis supplied to the coil 3 as a plasma source so as to generate plasma inthe reaction chamber 15. Further, high-frequency power 200 W is suppliedto the lower electrode 14 such that boron is implanted in the vicinityof the surface of the silicon substrate 13. Here, plasma exposed to thesilicon substrate 13 is mixture gas plasma of B₂H₆ and He (B₂H₆/Heplasma). Moreover, the mixture ratio of B₂H₆ and He can be changed bychanging a ratio of flow rates of He gas and B₂H₆ gas flowing in themass flow controllers 4 and 5.

If a bias is applied by irradiating the mixture gas plasma of B₂H₆ andHe (B₂H₆/He plasma) onto the silicon substrate, there is a time whendoping and sputtering of boron are saturated (balanced). Further, inthis embodiment, it can be seen that the saturation time iscomparatively long and ease to stably use. That is, if plasmairradiation starts, a dose is initially increased, but a time at whichthe dose is made substantially uniform without depending on a timevariation is continued. In addition, if the time is further increased,the dose is decreased. Accordingly, the dose can be accuratelycontrolled through a process window of the time when the dose is madesubstantially uniform without depending on the time variation. Further,in-plane uniformity can be obtained by previously measuring, in thesurface of the silicon substrate, the time when the dose is made uniformand setting the doping time according to the latest start time.

Second Embodiment

Hereinafter, a second embodiment of the invention will be described withreference to FIG. 2.

An apparatus shown in FIG. 2 is referred to as a plasma doping apparatusB (PD apparatus B).

FIG. 2 is a schematic plan view of an impurity doping apparatus used inthe second embodiment of the invention. In FIG. 2, the impurity dopingapparatus uses a helicon plasma device, and thus, in this device,B₂H₆/He gas and He gas are supplied through mass flow controllers 24 and25.

Here, the impurity is also doped into a silicon substrate (sample) 33placed on a sample table 34 in a reaction chamber 35. High frequency issupplied to a coil 23 by a high-frequency power supply 21, and then theB₂H₆/He gas and He gas to be supplied through the mass flow controllers24 and 25 are plasmized.

In this apparatus, the mass flow controllers 24 and 25 are controlledwith high accuracy, and the mixture ratio of B₂H₆/He is controlled.Accordingly, the concentration of the gas plasma can be controlled suchthat the dose is mad uniform with no time dependency.

Example 1

Plasma doping is performed on a 200 mm substrate using the PD apparatusB described in the second embodiment with reference to FIG. 2 while thedose of boron and the plasma doping time are changed.

FIG. 3 shows the measurement result of the relationship between the doseof boron and the plasma doping time at this time. The vertical axisrepresents the dose and the horizontal axis represents the plasma dopingtime.

If a bias is applied by irradiating the mixture gas plasma of B₂H₆ andHe (B₂H₆/He plasma) onto the silicon substrate, there is a time whendoping and sputtering of boron are saturated (balanced). Further, inthis embodiment, it can be seen that the saturation time iscomparatively long and ease to stably use. That is, if plasmairradiation starts, a dose is initially increased, but a time at whichthe dose is made substantially uniform without depending on a timevariation is continued. In addition, if the time is further increased,the dose is decreased. Accordingly, the dose can be accuratelycontrolled through a process window of the time when the dose is madesubstantially uniform without depending on the time variation. Further,in-plane uniformity can be obtained.

With this phenomenon, for example, it is verified that the dose of boroncan be set to 2.62E15 cm⁻² at accuracy within 1σ32 1%. If plasma dopingis performed with the B₂H₆/He gas concentrations 0.2%/99.8% at V_(DC) 60V, source power 1500 W, and a pressure 0.9 Pa, a change in the dose from45 seconds to 60 seconds is negligibly 0.01E15 cm⁻² since the dose at 45seconds is 2.62E15 cm⁻² and the dose at 60 seconds is 2.63E15 cm⁻², asindicated by a curve b of FIG. 3. At this time, the dose is increased ina small increment per unit time. The rate is very slow, that is,(2.63E15−2.62E15)/(60−45)=6.7E11 cm⁻²/seconds. That is, the dose isstable with respect to the time variation.

A difference (±3σ) between the maximum and the minimum of the tolerabledose such that the condition 1σ=1% is established when 2.62E15 cm⁻² issought is 2.52-2.68E15 cm⁻², that is, approximately 1.5E154 cm⁻². Duringthe doping time between 45 seconds to 60 seconds, the dose is changedonly at a slow rate of 6.7E11 cm⁻²/sec. Accordingly, it is expected thatthe dose control of 1σ=1% aimed at 2.62E15 cm⁻² can be performed. Thisis because the control of the doping time to be determined by theapparatus is in an order of 100 milliseconds and the doping time isshifted by 1 second at most.

Here, the dose is estimated from sheet resistance after annealing isperformed at 1100° C. and for 3 minutes, and a difference between sheetresistances of samples during the doping time between 45 seconds to 60seconds is negligibly 107.4 to 107.0 ohm/sq and 0.4 ohm/sq. The smallchange in the dose for a long time of 15 seconds is a remarkablediscovery. Though described above, in view of the apparatus control, atime shift when plasma doping is repeatedly performed for 50 seconds ishundreds microseconds at most. Accordingly, since it is sufficient topay attention to a shift of approximately 50 seconds±0.5 seconds, a dosecontrol method is very stable and has high controllability.

From FIG. 3, it can be seen that, when the B₂H₆/He gas concentration is0.025%/99.975%, there is a time range in which doping and sputtering ofboron are balanced in the vicinity of 60 seconds. When the B₂HE/He gasconcentration is 0.1%/99.9%, the time range exists in the vicinity of 60seconds. When the B₂H₆/He gas concentration is 0.2%/99.8%, the timerange exists in the vicinity of 45 to 60 seconds. When the B₂H₆/He gasconcentration is 0.5%/99.5%, the time range exists in the vicinity of 60to 70 seconds. In addition, when the B₂H₆/He gas concentration is0.6%/99.5%, the time range exists in the vicinity of 60 to 100 seconds.In these vicinities, in view of the apparatus control, a time-variantchange in the dose is very small, and the dose can be controlled withhigh accuracy. This can be explained by the same logic as a case wherethe B₂H₆/He gas concentration is 0.2%/99.8%.

FIG. 4 shows, with respect to the time variation from the experimentresult of FIG. 3, the arrangement result of the relationship between thedose B and the B₂H₆/He gas concentration when the time variation issaturated. The vertical axis represents the saturated dose and thehorizontal axis represents the B₂H₆/He gas concentration. As a result,the B₂H₆/He gas concentration and the saturated dose have a one-to-onerelationship. From the above description, the following can be verified.First, when the B₂H₆/He gas concentration is changed, the level of thedose in which the change in the dose with respect to the time variationis made substantially uniform can be changed. Further, the B₂H₆ gasconcentration is adjusted such that the level of the dose in which thechange in the dose with respect to the time variation is madesubstantially uniform becomes a desired dose. In addition, the plasmadoping time is adjusted to the time region where the change in the dosewith respect to the time variation is made substantially uniform, andthus the dose can be accurately controlled to a desired value.

In contrast, only by adjusting the B₂HG gas concentration, stabilityagainst the time shift is lacking. Further, in a region other than thetime region where the change in the dose with the time variation is madesubstantially uniform, only by adjusting the plasma doping time,stability against the time shift is lacking. In addition, the timeregion where the change in the dose against the time variation is madesubstantially uniform varies by the B₂H₆ gas concentration. Accordingly,it is necessary to adjust the time region according to each B₂H₆ gasconcentration. When this adjustment is performed, while the dose can beaccurately adjusted at a certain B₂H₆ gas concentration, stabilityagainst the time shift may be lacking at a different B₂H₆ gasconcentration.

FIGS. 5 and 6 show the results of annealing on the substrate at 1075° C.and for 20 seconds after plasma doping prepared using the dose controlmethod. Sheet resistance is measured at 81 places in the surfaceexcluding an end 5 mm of the 200 mm substrate. This has a feature inthat the plasma doping time at which the dose is made uniform withrespect to the time variation and the B₂H₆/He gas concentration areused. Here, a simple description will be given with reference to FIGS. 3and 4. If the B₂H₆/He gas concentration is appropriately set withrespect to a predetermined bias, as shown in FIG. 3, it is possible toform a time range where the B dose is not almost changed and saturatedwith respect to the change in the plasma doping time. The B dose to besaturated can be changed when the B₂H₆/He gas concentration is changed,as shown in FIG. 4. That is, the dose can be controlled. Here, theplasma doping conditions used in FIGS. 5 and 6 are the plasma dopingconditions indicated by arrows a and b in FIGS. 3 and 8. Symbol arepresents FIG. 5 and symbol b represents FIG. 6.

FIG. 5 shows Rs uniformity when the B dose is adjusted to 1.63E15 cm⁻²by setting the B₂H₆/He gas concentration 0.1%/99.9% and the plasmadoping time 60 seconds using the dose control method. The average of Rsis 194.0 ohm/sq, and uniformity is 2.25% at 1σ. In case of a sample thatis prepared at a time at which the dose is not saturated, uniformity isapproximately 5% to 10% at 1σ, and thus a uniform layer is not formed.This is one of effects according to the selection of the saturation timeof the dose.

FIG. 6 is a diagram showing the measurement results of the relationshipbetween the gas concentration and the time when the B dose is adjustedto 2.62E15 cm⁻² by setting the B₂H₆/He gas concentration 0.2%/99.8% andthe plasma doping time 45 seconds using the dose control method. Theaverage of Rs is 147.9 ohm/sq, and uniformity is 2.42% at 1σ. As such,in case of the dose different from FIG. 5, good uniformity of 2.5% orless can be reproduced.

In general, as for uniformity, as the value of 1σ is smaller, a degreeof technical difficulty in improving uniformity is rapidly increased.That is, of a degree of difficulty in improving uniformity from 10% to5% and a degree of difficulty in improving uniformity from 5% to 2.5%,the latter case is still more difficult. As for uniformity of 5% or morewhen the invention is not applied, with the application of theinvention, uniformity of 2.5% or less is easily obtained. Thisrepresents validity of the invention.

Example 2

Next, as Example 2 of the invention, plasma doping is performed on a 300mm substrate using the PD apparatus A shown in FIG. 1 while the dose ofboron and the plasma doping time are changed.

FIG. 7 shows the measurement results of the plasma doping time, and thedose of boron and in-plane uniformity. It can be seen that the dose ofboron starts to be saturated with respect to the time variation atapproximately 30 seconds. Further, the dose of boron and in-planeuniformity exhibit good values when 30 seconds lapse, that is, when theplasma doping time reaches 60 10 seconds.

FIG. 8 shows an in-plane distribution of sheet resistance after boron isdoped into the 300 mm substrate shown in FIG. 7 by plasma doping andthen annealing is performed at 1075° C. and for 20 seconds. Sheetresistance is measured at 121 places in the surface excluding an end 3mm of the 300 mm substrate. FIG. 9 shows the distribution of sheetresistance on the vertical axis passing through the center of thesubstrate among the in-plane distributions in FIG. 8. Further, FIG. 10corresponds to FIG. 9 and is a table showing normalized values of sheetresistance obtained by dividing sheet resistance by an average value ineach surface of the substrate.

As such, the dose is not changed according to the change in the plasmadoping time, and after a while, in-plane uniformity is also improved.This may be because the dose is not changed with respect to the changein the plasma doping time, and after a while, a dose in a portion wherethe dose in the surface of the substrate is small follows a dose in aportion where the dose is saturated.

Meanwhile, if a time immediately after the dose is not change withrespect to the change in the plasma doping time is set to the plasmadoping time, the dose in the portion where the dose in the surface ofthe substrate is small does not follow the dose in a portion where thedose is saturated. In this case, in-plane uniformity is not sufficient.

That is, even though the dose control is sufficient, in order to securein-plane uniformity, it is necessary to more optimally set the plasmadoping time.

Next, a mechanism of the invention will be described with reference toFIGS. 9 and 10. In FIGS. 9 and 10, sheet resistances after 7 seconds (7sec), 30 seconds (30 sec), and 60 seconds (60 sec) are shown. In aportion (a portion from 150 mm to 0 mm on a horizontal axis of FIGS. 9and 10) where a large amount of boron is initially doped, the dose iscomparatively rapidly saturated as time lapses. To the contrary, in aportion (a portion from 75 mm to 150 mm on a horizontal axis of FIGS. 9and 10) where boron is not doped so much at the beginning of plasmadoping, it takes comparatively much time until the dose is saturated.

However, if the does starts to be saturated and time lapses, in theportion where a large amount of boron is initially doped, the dose issaturated and boron is not doped any more. Meanwhile, in the portionwhere boron is not initially doped so much, the dose reaches thesaturation. Accordingly the difference becomes smaller. For this reason,in FIGS. 9 and 10, when doping is performed for 60 seconds, a variationwidth of sheet resistance on the vertical axis is small. Accordingly,in-plane uniformity of sheet resistance can be improved. The methodaccording to the invention is a unit effective to secure in-planeuniformity upon plasma doping. Simultaneously, as described above, thedose can be controlled.

Moreover, under the annealing condition at 1075° C. and for 20 secondsused in Examples 1 and 2, the distribution of sheet resistance may beregarded as the distribution of the dose. This is because the dose andsheet resistance has a one-to-one relationship. Under the annealingcondition at high temperature and for a comparatively long time, it canbe estimated that the impurity is almost electrically activated. This isconsidered as a reason for the one-to-one relationship.

Next, an activation step will be described. This is common to, notdepend on, the PD apparatus. Upon annealing, the silicon substratesupplied with the impurity ions is placed on the sample table of anannealing device. Then, laser light that is emitted from an infraredlaser and reflected by a mirror is irradiated onto the surface of thesilicon substrate, such that the surface of the silicon substrate isheated and activated.

Moreover, in the activation step, as an activation processing chamber, aflash lamp processing chamber may be used. The flash chamber processingchamber includes a chamber, a sample table, a window, and a flash lamp.The silicon substrate supplied with the impurity ions is placed on thesample table, and reflected light from the flash lamp is irradiated ontothe surface of the silicon substrate, such that the surface of thesilicon substrate is heated and activated.

Moreover, in the above embodiment, in the activation step, the flashlamp processing chamber is used as the activation processing chamber.However, a tungsten halogen lamp that is used in a semiconductor factoryfor mass production at present may be used.

In the above-described embodiments of the invention, among theapplication scopes of the invention, only some of various variations ofthe configuration, shape, and arrangement of the processing chamber areillustrated. Of course, various variations, which still fall within thescope of the invention, other than illustrations can be considered.

Further, although a case where the sample is a semiconductor substrateformed of a silicon substrate has been described, the invention can beapplied when various different samples are processed. For example, theinvention can be effectively applied to a stained silicon substrate andan SOI substrate. This is because the substrates are the same as thesilicon substrate in view of only the surface shape as viewed from theplasma. In addition, the invention can be effectively applied to a FinFET. In case of Fin FET, in general, the structure is in an order of 1μm or less. A sheath width of the plasma is in an order of 1 mm or more.Accordingly, when only the surface shape as viewed from the plasma isconsidered, the structure of Fin FET is negligibly small. This is thesame as the silicon substrate.

Further, although a case where the impurity is boron has been described,when the sample is the semiconductor substrate formed of silicon, inparticular, when the impurity is arsine, phosphorous, boron, aluminum,or antimony, the invention can be effectively applied. This is because ashallow junction can be formed in a transistor portion.

Further, the invention can be effectively applied when the concentrationof gas containing an impurity is low. In particular, the invention canbe effectively applied to a plasma doping method in which the dose needsto be controlled with high accuracy.

Further, a case where, in the plasma doping step, the gas supplied tothe reaction chamber is gas containing a doping material has beendescribed. However, the invention can be applied to a case where the gasto be supplied to the reaction chamber does not include the dopingmaterial, and the doping material is generated from a solid impurity.That is, the invention can be effectively applied to a case where asolid containing impurity atoms is placed in the reaction chamber,plasma, such as He or the like, is excited, and the impurity atoms areplasmized and doped by plasma doping.

Further, when the plasma doping is performed, it is preferable that thetreatment be performed in a state where a temperature of an inner wallof the reaction chamber coming into contact with the plasma is keptsubstantially constant. This is because, if the temperature of the innerwall of the reaction chamber is changed during the treatment, anattachment possibility of the impurity ions at the temperature of theinner wall is changed, and the number of impurity ions to be emittedfrom a thin film containing the impurity ions stuck to the inner wallinto the plasma is changed, which cause the change in the dose per unittime. As a method of keeping the temperature of the inner wall of thereaction chamber, a method of heating the inner wall by a heater or amethod of cooling the inner wall through the circulation of arefrigerant can be appropriately selected.

Further, when the concentration of gas containing the impurity in thereaction chamber is adjusted, a method that adjusts the gas supplyamount so as to directly adjusts the concentration itself, a method thatlowers the temperature of the inner wall of the reaction chamber andeduces a predetermined impurity so as to lower the concentration of theimpurity, a method that lowers the temperature of the inner wall of thereaction chamber and suppresses the eduction of a predetermined impurityso as to keep the concentration of the impurity, or a method thatadjusts the temperature of the inner wall of the reaction chamber so asto adjust the dose can be used. Further, with a feedback function, theconcentration control can be performed while the temperature control ofthe inner wall of the reaction chamber.

Further, when plasma doping is performed, the concentration of gascontaining the impurity ions may be lowered during the treatment. Anappropriate method for this case will be described.

First, in a state where the concentration of gas containing the impurityatoms is high, plasma doping is performed. At this time, the dose perunit time at the beginning of the treatment is set high.

Next, in a state where the concentration of gas containing the impurityatoms is low, plasma doping is performed. Then, the plasma dopingtreatment is stopped in a time range in which the dose is mad uniformwith no time dependency. In such a manner, compared with a case wherethe treatment is performed in a state where the concentration of gascontaining the impurity ions from the beginning, the total processingtime can be reduced.

In this case, a method that lowers the concentration of gas by raisingthe temperature of the inner wall of the reaction chamber, increasingthe concentration of gas including the impurity to the maximum, thenlowering the temperature of the inner wall of the reaction chamber, andsubsequently promoting the eduction of the impurity to the inner wallmay be effectively used.

According to the plasma doping method, it is possible to realize aplasma doping method that can control an impurity doping amountprecisely and economically and can form a shallow impurity diffusionregion. In addition, the plasma doping method of the invention can beapplied to the use, such as an impurity doping process of asemiconductor or manufacturing of a thin film transistor used in liquidcrystal or the like.

1. A plasma doping method for irradiating plasma of gas containing animpurity onto a substrate so as to form an impurity doped region in asurface of the substrate, comprising the steps of: determining a plasmadoping time range in which a dose of the impurity to be doped into thesubstrate is made substantially uniform irrespective of variation inplasma doping time; and after the step of determining the plasma dopingtime range, plasma-doping the impurity into the substrate using apredetermined plasma doping time within the plasma doping time range. 2.The plasma doping method according to claim 1, further comprising a stepof: before the step of plasma-doping, determining a concentration of thegas containing the impurity used in the step of plasma-doping, bycalculating relationships between a concentration of the gas containingthe impurity and the uniform dose which is made substantially uniform,irrespective of variation in plasma doping time, with respect to aplurality of concentrations.
 3. The plasma doping method according toclaim 1, wherein the step of determining the plasma doping time rangecomprises a step of respectively calculating plasma doping time ranges,in which the dose of the impurity to be doped into the substrate is madesubstantially uniform irrespective of variation in plasma doping time,at a plurality of portions in the surface of the substrate; and thepredetermined plasma doping time is determined based on a latest plasmadoping time range among the respective plasma doping time ranges.
 4. Theplasma doping method according to claim 1, wherein a diameter of thesubstrate is 200 mm or more; and the predetermined plasma doping time isdetermined such that uniformity represented by 1σ of the dose of theimpurity in the surface of the substrate becomes 2.5% or less.
 5. Theplasma doping method according to claim 4, wherein the diameter of thesubstrate is 300 mm or more.
 6. The plasma doping method according toclaim 1, wherein the plasma of the gas containing the impurity is amixture gas plasma of molecules (B_(n)H_(m)) having boron atoms andhydrogen atoms.
 7. The plasma doping method according to claim 1,wherein the plasma of the gas containing the impurity is a mixture gasplasma of B₂H₆ and helium.
 8. The plasma doping method according toclaim 7, wherein a B₂H₆ gas concentration in the mixture gas plasma ofB₂H₆ and helium is in a range of 0.01% (inclusive) to 1% (inclusive). 9.The plasma doping method according to claim 7, wherein a B₂H₆ gasconcentration in the mixture gas plasma of B₂H₆ and helium is in a rangeof 0.025% (inclusive) to 0.6% (inclusive).
 10. The plasma doping methodaccording to claim 1, wherein the plasma of the gas containing theimpurity is mixture gas plasma of BF₃ and helium.
 11. The plasma dopingmethod according to claim 1, wherein the substrate is a siliconsubstrate.
 12. The plasma doping method according to claim 1, whereinthe step of plasma-doping includes: a first doping step of plasma dopingby setting the gas containing the impurity to a first concentration; andafter the first doping step, a second doping step of plasma doping bysetting the gas containing the impurity to a second concentration, thesecond concentration being larger than the first concentration.